Multicolor printing processes typically require the sequential printing of a plurality of superposed single color ink layers. When high quality image reproduction is desired, it is important to avoid a previously applied ink layer mixing with a subsequently applied ink layer. Such layer mixing typically results in undesirable color rendition.
This problem has been addressed in a number of different ways. The simplest way to prevent undesirable color mixing is to dry each applied ink layer prior to the application of a superposed next ink layer. However, this method suffers a major disadvantage, in that it requires complete drying after applying each ink layer. Drying takes time and energy to accomplish, and, as a result, productivity is reduced and production costs increase.
In an effort to speed up the printing process, wet trapping was developed. Wet trapping is a process whereby the ink layer deposited or applied at each inking station is not dried before the next ink layer is deposited thereover to produce a coloristic or visual effect. To implement wet trapping, it is important that the tack characteristics of the superposed ink layers be different.
Wet trapping is not a serious problem in offset printing, because the viscosity of the inks used in offset printing, ranges from 20,000 to 100,000 cps. Such high viscosity inks exhibit a wide range of tack characteristics that can be used to effect wet trapping without the need to dry the ink layers between inking stations.
In recent years, a form of printing that permits printing on various kinds of substrates, varying from cardboard to polyethylene to metal, has become widely accepted. This printing method is known as flexography.
Flexography employs a resilient printing plate having raised portions, which are coated with an ink and pressed against a substrate to transfer the ink to the substrate. In flexography, ink is transferred from a reservoir to the printing plate's raised surface through an intermediate transfer roll known in the art as an anilox roll. The anilox roll surface is covered by a plurality of tiny ink wells that fill with ink from the reservoir and transfer it to the flexographic printing plate. Obviously high quality printing requires that the flexographic printing plate surface be inked uniformly and consistently. This in turn requires that the anilox roll cells be small and that all of the anilox cells be filled each time with ink from the reservoir to substantially the same level.
Such requirement poses limitations on the fluidity or viscosity of the ink. A viscous ink will not be picked up as uniformly or consistently by the anilox roll and the flexographic printing plate surface will not be inked uniformly. The result has been that inks suitable for flexographic applications typically have viscosities under 2,000 cps, preferably less than 400 cps.
Current regulations regarding solvent emissions have resulted in the development of inks suitable for use in flexography that are energy curable. Such inks contain little or no solvent, and are fixed to the substrate not by drying but by curing via actinic radiation, such as ultraviolet light or electron beam. Their tack is very low and cannot be adequately measured with conventional instruments. Their viscosities are in the range of about 30 to 50 cps. While such viscosity range results in superior flexographic printing, energy-curable inks for flexographic applications exhibit very low tack, cannot be tack rated, and need be to cured between inking stations to prevent back transfer and mixing from the printed ink on the substrate to the inking rolls of subsequent stations. Such inter-station curing is expensive, as it requires substantial equipment modification. Such curing is also undesirable from a manufacturing stand point, as it increases the time required between the deposition of a subsequent ink layer in order to allow for curing of the previously deposited ink layer, thereby slowing down the printing process.
Wet trapping has also been proposed in flexographic printing based on the recognition that when depositing superposed multiple layers of ink, mixing will not occur if each layer is deposited over a layer having a higher viscosity than the newly deposited layer. The highest viscosity layer traps, so to speak, the second layer without mixing with or transfer of the underlying layer. However, with the range of viscosities available for flexographic printing inks, it is impractical to implement wet trapping using constantly decreasing ink viscosities for each layer that are sufficiently different from each previously applied layer viscosity in order to effect wet trapping, particularly as the number of applied layers increases. In essence, one runs out of available ink viscosities to implement wet trapping.
U.S. Pat. No. 5,690,028 attempts to solve the above mentioned problem of limited available ink viscosity range using a method of wet trapping in a multicolor printing application using energy curable inks. However, here the energy curable inks are heated before being applied to a substrate, and are applied to the substrate at a temperature that is higher than the previously applied ink layer. Because the temperature of the previously applied ink layer on the substrate is cooler than the heated ink, the viscosity of the previously applied ink layer is lower than the viscosity of the applied ink. This viscosity differential causes the lower viscosity ink to unilaterally transfer onto the higher viscosity ink and prevents both back trapping and ink blending. This method requires substantial modification to existing printing press equipment to provide for heating units in each inking station before the ink is applied to the substrate. Moreover, as the number of stations increases, so must the ink temperature in the successive inking stations. Thus, it may be necessary to apply cooling to the substrate, or the printing speed may have to be reduced, in order to prevent having to increase the ink temperature to levels that may adversely affect its properties.
U.S. Pat. No. 6,772,683 describes a method for flexographic printing of multiple superposed ink layers on a substrate without prior curing of the earlier printed inks. The method involves applying onto a substrate ink layers having a non reactive diluent, and then evaporating at least a portion of the non-reactive diluent in the applied ink layer, thereby increasing the viscosity of the applied ink layer. Then a subsequent ink layer is applied. The previous layer, as a result of the evaporation of the diluents, has an increased viscosity. Thus the newly applied ink layer has a viscosity lower than the increased viscosity of the previously applied ink layer.
Similarly, U.S. Pat. No. 6,772,682 describes a process whereby wet trapping of Energy Curable (Ultraviolet or Electron Beam) inks can be achieved by loss of a fugitive diluent (water) present at levels from 5% to 50%. The loss of a small amount of water in an EB flexo ink can cause a many-fold rise in viscosity. This causes printing process instability.
U.S. Pat. No. 7,329,438 describes how to make a wet coating printed over wet (or dry) ink smoother (higher gloss) through the application of roller pressure from a mirror surface roller with very low surface tension. However, it does not teach how to make the wet coating trap over the wet ink in the first place.
United States Patent Application 2007/0289459 describes overcoming the need for a fugitive solvent in an EB flexo system by partially curing the first down ink before the 2nd down ink is trapped over it. This, of course, requires an intermediate curing mechanism.
United States Patent Application 2010/0242757 further extends radiation curable (EB) wet trapping methods for printing inks to the gravure printing technology. It features inks that are dried/cured by ovens or IR heaters and are hard enough to pass between printing decks without being marred by turn bars or other face contact point. However, this disclosure still relies on a fugitive solvent to achieve wet trapping, which involves unnecessary complexity.
A solvent-free and water-free wet trapping technique would provide a novel solution to wet-on-wet printing of energy curable liquid inks that does not require partial interstation cure or full inter-station cure with actinic radiation. A printing technique that does not require the addition of a volatile, evaporating component to achieve the viscosity differential required for wet trapping could have commercial viability as a more “foolproof” printing application technology. Such a technique would avoid the logistical and technical challenges of providing an ink that is stable over a range of printing and evaporation conditions, as well as over a sufficiently broad water content range.